A slewing-type working machine such as hydraulic excavator includes a lower travelling body, an upper slewing body, and a slewing mechanism for slewing the upper slewing body above the lower travelling body. For example, Japanese Unexamined Patent Publication No. 2002-302972 discloses a slewing mechanism that includes a ring gear having a plurality of teeth arranged along an inner circumference thereof and a pinion meshable with the plurality of teeth of the ring gear. The ring gear is fixed to the lower travelling body, and the pinion is supported by the upper slewing body. The pinion is driven by a drive means such as hydraulic motor disposed on the upper slewing body to rotate while meshing with the ring gear. The center of the ring gear coincides with the slewing axis of the upper slewing body. Therefore, the movement of the pinion in a circumferential direction of the ring gear while rotating causes the upper slewing body to be slewed with respect to the lower travelling body.
The slewing mechanism includes a grease bath. The grease bath is shaped in an annular recess extending along the inner circumference of the ring gear, storing grease for lubricating a meshing portion between the ring gear and the pinion. The pinion is supported at a position where a part of the pinion is immersed in the grease in the grease bath. The grease in the grease bath is drawn into the rotating pinion to be supplied to the meshing portion between the pinion and the ring gear.
The grease in the grease bath, however, may be contaminated and deteriorated by water particles having entered the slewing mechanism or by metal powder produced from components of the slewing mechanism. The deteriorated grease is likely to hinder the operation of the slewing mechanism. For example, water particles mixed in the grease in the grease bath significantly reduce the viscosity of the grease while being stirred by the pinion in rotation. The reduction in the viscosity inhibits the grease from sufficiently adhering to the pinion and the ring gear, thus deteriorating the lubrication effect of the grease. This may promote wear and damage of the ring gear and the pinion, hindering the smooth operation of the slewing mechanism. Besides, the water particles having entered the grease bath may be dispersed by the rotating pinion or may splash out from a space between meshed teeth of the ring gear and the pinion. The thus splashed water may then strongly hit a sealed portion of the slewing mechanism and enter a movable portion disposed inside the slewing mechanism to thereby inhibit the slewing mechanism from smooth operation.
To solve the problem, it can be considered to detect a contaminant having a potential for deteriorating the grease in the grease bath to accurately recognize the state of deterioration of the grease, specifically, for example, providing a sensor 70 for detecting a water particle (contaminant) at a bottom portion 20a of a grease bath 20, as shown in FIG. 10.
However, even if water particles accumulate on the top surface of the grease G, the grease G filled in the grease bath 20, having a high viscosity, is likely to prevent the water particles from reaching the bottom portion 20a of the grease bath 20. Furthermore, a portion of the grease G near the bottom portion 20a of the grease bath 20, being hard to stir by a pinion 17, may form a layer of non-deteriorated grease.
For the above reasons, it is difficult to detect a contaminant even though the sensor 70 is disposed at the bottom portion 20a of the grease bath 20.